A booster amplifier is used to amplify wireless communication signals. More particular, a signal booster is a device that automatically receives, amplifies, and retransmits on a bidirectional or unidirectional basis, the signals received from base, fixed, mobile, or portable stations, with no change in frequency or authorized bandwidth.
Such signals boosters are used in transport vehicles like cars, trains and so on. However, the use of signal boosters is not limited to transport vehicles. It can also be used within buildings for example. Signal boosters comprise a first antenna and a second antenna, wherein the first antenna is arranged within the transport vehicle and wherein the second antenna is arranged at the outside of the transport vehicle. The first antenna is used for sending and receiving communication signals form a mobile, wherein the second antenna is used for sending and receiving communication signals from a base station. Thus, the mobile is connected to the base station though the signal booster. It is very advantageous that the transmit power of the mobile is far below from its maximum transmit power, wherein it is ensured at the same time that a stable connection is established between the mobile and the base station. This connection is independent of any influence from an electromagnetic shielding the transport vehicle may comprise.
In general, a repeater is also some kind of signal booster. However, a repeater is able to amplify a signal with a much higher gain than a signal booster.
Unfortunately, it is possible that the use of a signal booster disrupts cellular systems. This disruption may not be limited to the network the signal booster uses, but could also be spread to other neighboring networks. A power overload situation may occur. In a result, other devices may be disconnected. A signal booster may also increase the noise floor, which decreases the sensitivity of the base station and therefore reduces the covered area for all devices. These effects are often a result when the signal booster begins to self-oscillate. Furthermore, this could also result in a poor reception by the base station inhibiting a mobile to establish a communication at all.
To prevent these malfunctions, any occurring oscillations within a signal booster have to be detected and as a result the amplification has to be reduced or disabled at all. In order to obtain an approval for operating a booster device, the FCC (Federal Communication Commission) has issued guidelines that any booster device has to comply with. Reference is made to FCC Title 47, Chapter I, Subchapter B, Part 20, § 20,21 (Signal boosters) dated Feb. 11, 2015 which is herein incorporated by reference.
The aforementioned problem has also been partly addressed by the international patent application WO 2013/028921 A1. This application describes a method for detecting self-oscillations within a signal booster. The method is based on a power measurement of the communication signal. The communication signal is attenuated by a certain factor larger than zero. This factor may also be applied when the signal booster amplifies signals from the base station or from the mobile and transmits them to the mobile or the base station. A normal operation gain of the signal booster would be achieved. Then a first signal power is measured. If any oscillations occur, these oscillations are contained within the measurement result. Afterwards, the attenuation is increased so that the signal level is reduced and another signal power is measured this time. By increasing the attenuation, the oscillations are reduced much more than the (useful) signal itself. By determining a difference between the measured signal powers, it can be assessed whether the booster amplifier oscillates or not.
The FCC demands that an oscillation has to be detected at latest if the oscillation reaches a level of 12 dB above the noise floor.
The power of an oscillation depends on the isolation between the input port and the output port of the signal booster device (e.g. antenna ports). If the isolation is higher, the power of the oscillation is lower and vice versa.
The signal booster disclosed in the WO 2013/028921 A1 is only able to detect oscillations if the isolation between the input port and the output port is below a respective isolation level.
Therefore, the problem addressed herein is that of describing and/or creating a method and a signal booster device fulfilling the FCC requirements with respect to signal boosters in a reliable and repeatable manner. The signal booster device according to the present allows detecting oscillations at an isolation level that is higher than the one of the signal booster disclosed in the WO 2013/028921 A1. The occurrence of oscillations can therefore be detected sooner.